This application is a continuation-in-part of copending application Ser. No. 045,118, now abandoned filed May 4, 1987, assigned to the same assignee as the present invention and incorporated herein by reference.
The present invention relates to a method for making 9,10-dithiaanthracene-2,3,6,7-tetracarboxylic acid dianhydride, or derivatives, referred to hereinafter as "thianthrene dianhydride" and to polyimides having glass transition temperatures in excess of 400.degree. C.
Prior to the present invention, aromatic polyimides were generally recognized as organic materials having superior solvent resistance and temperature resistance. Aromatic polyimides can generally be made by intercondensing aromatic dianhydrides with aliphatic or aromatic diamines. One technique for improving the glass transition temperature (T.sub.g) of aromatic polyimides is by intercondensing aromatic dianhydrides with aromatic diamines. Further improvements can be achieved in aromatic polyimide properties by utilizing a particular aromatic dianhydride to provide a polyamic acid which is convertible to a polyimide having higher temperature resistance and solvent resistance upon heating.
The present invention is based on the discovery that aromatic polyimides having glass transition temperatures in excess of 400.degree. C. can be made by intercondensing one or more aromatic diamines, such as methylene dianiline or metaphenylene diamine with thianthrene dianhydride having the formula, ##STR1## where R, and R.sup.1 are selected from the same or different C.sub.(1-8) alkyl radicals, C.sub.(1-8) alkoxy radicals, and C.sub.(6-14) aryl radicals, and a a and b are whoel numbers equal to 0 or 1.